Phased array antenna broadcast systems, such as those used for mobile telephone and other data communication systems, take advantage of the phase differential that occurs according to the direction of coherent propagating energy. For example, in a simple array of two closely spaced antenna elements lying in a plane and both facing forward, an incoming signal coming straight from the forward direction would be received at the same time at both elements, resulting in signals at each element having the same phase, which are referred to as “in-phase.” But if the energy approaches the elements at an angle, the two elements receive the energy at different times, resulting in a phase differential or “shift” between the two signals. This is similar to ocean waves arriving at a beach. If the wave comes straight in to shore, the wave washes upon the beach at the same time along the beach. If the wave is coming in at an angle relative to the beach, however, it arrives first in one spot and then progressively arrives down the beach at later times.
A similar phenomenon is at work in phased array antenna systems. Since the propagating electromagnetic energy reaches the nearest antenna element first, the direction of the incoming energy can be determined by detecting the phase differential. Similarly, energy emitted from the antenna may be pointed in a particular direction by controlling the phase angles of the signals emitted from the antenna elements. For example, a directional “beam” may be formed by emitting signals from the antenna elements with coordinated phase delays, which causes the emitted energy to add up constructively in a desired beam direction while partially or completely canceling out in all other directions. It is common to steer a coherent beam created in this manner by controlling programmable phase and gain control devices at each antenna element in a coordinated manner. For example, a single beam formed by a phased array may be controlled to periodically sweep across the antenna's angular coverage, to track an intended receiver, to sweep or track while avoiding a known signal, or to achieve other objectives. This conventional beam steering system uses a single controllable phase and gain control device for each antenna element and a beam steering computer to create and control the beam.
It is also conventional to use a phased array antenna system to simultaneously broadcast multiple beams having different pointing directions. For example, rather than steering one beam to sweep across the antenna's angular coverage, as described above, the phased array may be controlled to divide the antenna's angular coverage into multiple beams to broadcast data throughout the entire operational volume simultaneously. In addition, systems have been developed that can use a phased array antenna to broadcast different data in each beam. This is accomplished conventionally by dividing the signal emitted by each antenna element into separate beams for each user using separate phase and gain control devices at each antenna element for each user. That is, a separate beam is typically defined for each user containing that particular user's data. This typically requires a separate phase and gain control device at each antenna element for each user, and a separate data modulator for each user. In other words, the data signals for the individual users are conventionally formed by providing a separate data modulator and separate sets of antenna hardware at each antenna element for each user, which generally multiplies the required number of antenna hardware elements by the number of simultaneous users. This may be considered a “brute force” design technique due to the heavy dependence on antenna hardware to generate the desired beams.
However, applying this technology to a typical mobile telephone system would be prohibitively expensive and unwieldy. For example, the phased array antenna for a typical transmit base station might include 30 antenna elements that generate 10 simultaneous beams to serve 10,000 users. In this case, each of the 30 antenna elements would require 10,000 phase and gain control devices, resulting in 300,000 phase and gain control devices. The system would also require 10,000 data modulators to create the data signals for the 10,000 individual users. This approach would require 300,000 phase and gain devices and 10,000 data modulators, which would result in a system that is exorbitantly expensive, complex to construct, large in size, and heavy. Any one or more of these penalties may be critical for a particular application.
Alternatively, systems have been developed in which the data signals for the various users assigned to a particular beam are combined before they are supplied to the antenna elements. As a result, in this type of system each antenna element requires a separate phase and gain control device for each beam, rather than a separate phase and gain control device for each user. Although this design choice drastically reduces the number of phase and gain control devices, the system also requires a combiner for each beam. Referring to the previous example, this type of system would require 300 phase and gain control devices (i.e., one for each of the 10 beams at each of the 30 antenna elements), 10 beam combiners (i.e., one for each of the 10 beams) and 10,000 data modulators to create the data signals for the 10,000 individual users. Although the number of phase and gain control devices is greatly reduced, this type of system would still require a very large number of data communication hardware components, including 10,000 data modulators.
Accordingly, a need exists for improved methods and systems for broadcasting data using multiple beams with a phased array antenna system. In particular, a need exists for phased array antenna systems that can broadcast data to multiple users using multiple beams without dedicating a separate phase and gain control device to each user or each beam at each antenna element, and without requiring a separate data modulator for each user.